Thermal Conductivity Improvement of PCCs

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1		Nassima Radouane	--	1737	2022-11-27 13:01:31	\|
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This entry is adapted from the peer-reviewed paper 10.3390/en15218271

To overcome the long-standing disadvantages of PCMs, for instance, small values of thermal conductivity, liquid leakage, separation of phase, and the problem of supercooling, advanced phase change composites (PCCs) manufactured by chemical modifications or the incorporation of functional additives are essential to overcome these disadvantages and promote the large-scale application of PCMs.

phase change composites thermal conductivity enhancement energy conversion

1. Introduction

Faced with rising fuel sources use, the resultant CO₂ emissions and pollutants, and, moreover, a worldwide energy shortfall, the business community has adopted steps to shift energy consumption, moving in the direction of sustainable energy development ^[1]^[2]. In recent decades, there has been great interest in the extraction of renewable energy resources (solar, thermal energy,…etc.) and their application ^[3]^[4]. Nonetheless, using these energies is subject to the natural limits of intermittency, unpredictability and volatility, resulting in a lag in time, space, and intensity between supply and demand ^[5].

Thermal energy storage technologies (TESs) have been developed and have become an important part of the renewable energy storage infrastructure since the previous century in order to increase the efficiency of use and achieve the required control performance of thermal energy ^[6]. There are three types of thermal energy systems divided into sensible heat, latent heat, and thermochemical (see Figure 1) ^[7]^[8]. For this reason, phase change materials (PCMs) are a possible answer to the energy problem. They have the ability to store and release thermal energy when changing from one phase to another. In addition, latent heat storage utilizing phase change materials (PCMs) is one of the most effective and sought-after thermal energy storage systems due to its ease of operation and similar energy storage density ^[9]^[10].

Figure 1. Thermal storage methods and materials that are commonly used.

“Phase change material” refers to compounds and combinations with varying chemical characteristics ^[11]. Their common feature is a high latent heat of phase shift and an appropriate transition temperature. Figure 1 depicts the many forms of PCMs that exist. PCMs include four types, solid–liquid, solid–gas, solid–solid, and liquid–gas, according to their phase transition states. In turn, PCMs are classified into three major families according to their challenges and advantages ^[12]: organic PCMs, inorganic PCMs, and eutectic PCMs ^[13]. Each family is composed of several subfamilies. Figure 1 shows the different PCM families. To describe the differences between the three major families, Table 1 summarizes the benefits and drawbacks of the different phase change materials.

Table 1. Benefits and drawbacks of the different PCMs.

	Advantages	Disadvantages
Organic	Accessible in a wide range of temperature range No phase segregation Chemical stability Supercooling is negligible Compatibility with the building material Environmentally friendly, 100% recyclable	Low latent heat Low thermal conductivity thermal conductivity High volume expansion Flammable High cost compared to hydrated salts
Inorganic	High heat of fusion Availability High thermal conductivity high Low volume expansion Low cost Nonflammable	Segregation Loss of efficiency due to the cycles of fusion/solidification cycles Supercooling Corrosivity Dehydration due to thermal cycles
Eutectic	Volumetric storage density slightly higher than organic compounds. They have a net melting point similar to a pure substance	Only limited data are available on the thermodynamic properties. The use of these materials is very recent for the application of thermal storage

PCMs are employed in a variety of technologies, including building construction, solar energy storage, electronic component cooling, air conditioning systems, and the textile industry ^[14]^[15]. However, their application is limited due to the complexity of the solid–liquid phase change phenomenon, which is related to the dynamics of heat and mass transfers, the spatiotemporal distribution of the evolutions, and the specific behavior of PCMs during the phase change, which introduces an important singularity within the heat transfer model. Understanding and analyzing these transfer and phase change events is thus critical for designing a latent thermal energy storage system. The low heat conductivity of PCMs further limits their use. For example, kerosene waxes have a thermal conductivity of 0.15–0.4 W.m⁻¹.K⁻¹ ^[16]. In order to improve the thermal conductivity, it is necessary to add conducting fillers to this material to elaborate phase change composite (PCC). One of the ways used to remedy this problem is to impregnate the PCMs in a porous medium of high thermal conductivity. Furthermore, conventional PCMs have been modified with functional additives to enable PCCs functionalization and to broaden their potential applications. Metallic foams (MFs) are a porous medium with a metallic matrix that offer unique properties such as high porosity (porosity ranging between 0.8 and 0.98) and high thermal conductivity ^[17]. This qualifies them as a good option for enhancing heat transmission during PCM melting and solidification. To enhance the thermal conductivity of the pure phase change materials and hence their performance in different applications, PCMs were encapsulated in shells or inserted with highly conductive fillers such as carbon, metals, and ceramic materials ^[18]^[19]. Despite significant progress in the preparation and application of high-performance PCCs, advanced multifunctional PCC use is still in its early stages and requires additional exploration and research.

The goal of this study is to give a complete understanding of the most recent advances in the research and development of composite phase change material (cPCM) including material elaboration techniques and enhancement techniques of the thermal conductivity. Furthermore, special emphasis is placed on advanced applications of cPCMs, such as energy storage, energy conversion, and thermal management (Figure 2).

Figure 2. Form-stable PCCs integrated diagram, encompassing preparation processes, enhancement of thermal conductivity techniques, and their applications.

2. Thermal Conductivity Improvement of PCCs

To overcome the problem of low thermal conductivity of PCCs, many solutions have been explored and can be divided into two categories. The first solution consists of adding fins to the heat storage system to intensify the heat transfer during the solid–liquid phase change ^[20]^[21]. The second type of remedy is to improve the thermal conductivity of PCCs. This is achieved by adding conductive carbon-based additives such as carbon nanotubes, carbon fibers, and graphite or by adding conductive metal particles (TiO₂, aluminum nitride…) ^[20]^[22]. Improving PCC thermal conductivity can also be accomplished by impregnating PCCs with conductive porous materials ^[20]^[23].

2.1. Improvement of the Thermal Conductivity by Graphite Foams

High-porosity graphite foams present a promising solution for intensifying the low thermal conductivities of PCMs for heat storage. They have the advantages of low density; their high thermal conductivities, which reach 200 W.m⁻¹.K⁻¹; their thermal stabilities; their chemical compatibility with PCMs; and their resistance to corrosion ^[20]. Graphite foams are obtained by the PU foam method, the blowing method, and by compression ^[24]. They are subsequently soaked in liquid PCM to ensure the impregnation of the PCM into their pores. A graphite foam/PCM composite material combines the heat storage properties of PCMs with the high thermal conductivity of graphite. Many works in the literature have been devoted to the study of this type of composite.

Karthik et al. ^[25] studied the graphite foam/erythritol composite prepared by impregnation according to the protocol shown in Figure 3a. They highlight the filling of the pores of the graphite foam by the PCM. The results of the thermophysical characterization of the studied composite demonstrated that the thermal conductivity of graphite/erythritol is 3.77 W.m⁻¹. K⁻¹, which is five times higher than that of pure erythritol (0.72 W.m⁻¹.K⁻¹), with an impregnation factor of 75%. Tao et al. ^[26] used kerosene with a melting temperature of 51.4 °C and a thermal conductivity of 0.22 W.m⁻¹.K⁻¹ to fill the pores of graphite foam. They next cut the PCM-impregnated foam into 1–3 mm spheres. These spheres were sprayed with epoxy resin and pressed with 1 MPa pressure to make the composite PCC. According to the findings of this investigation, the introduction of graphite foam increased the thermal conductivity of the composite PCC to 4.98 W.m⁻¹.K⁻¹.

Another work by Karthik et al. ^[27] focused on the synthesis and assessment of the thermophysical characteristics of a graphite/paraffin composite. The results show that the thermal conductivity of the composite examined in the solid state is 2.6 W.m⁻¹.K⁻¹ and 1.8 W.m⁻¹.K⁻¹ in the liquid state. The introduction of graphite foam has therefore resulted in an increase in heat conductivity of 980% in the solid state compared to kerosene and 1530% in the liquid form.

2.2. Impregnation of PCMs in Metallic Foams

Metal foams with open porosity are promising possibilities for improving PCMs’ poor thermal conductivity. This is due to their high heat conductivity, up to 95% porosity, high permeability, and huge surface areas per unit volume ^[28]^[29]. They also benefit from low weight and good mechanical qualities such as stiffness ^[29]. These foams have a cellular structure formed by pores connected by metallic ligaments. The most-used foams, illustrated in Figure 3b–d, are the foams of copper, aluminum, and nickel ^[30].

The principle of using metal foams is based on the impregnation of their pores by a PCM to form a composite material. Prepared metal foam/PCM composites are attracting increasing interest in heat storage systems. They cover a wide range of applications that include electronic cooling ^[31], thermal management of Li-ion batteries ^[32]^[33], heat exchangers ^[34]^[35], and solar energy storage ^[36]^[37].

Figure 3. (a) Impregnation technique of erythritol–graphite composite foam preparation. Reproduced with permission from ^[38]. Metal foams of: (b) copper, (c) nickel, and (d) aluminum ^[30].

Many other methods are used to improve PCM’s thermal conductivity. Because of their excellent thermal conductivity and wide availability, carbon-based materials are attractive adjuncts for the fabrication of enhanced PCCs ^[39]^[40]. Carbon-based materials exhibit morphological structures that may be classed as 1D, 2D, and 3D, as shown in Figure 4 ^[41]. For instance, the 1D nanostructure was used for the first time by Wang et al. ^[42]; they created PEG-based PCCs with increased thermal conductivities using SWCNTs. The resulting PEG/SWCNT (2 wt%) PCC has a thermal conductivity of 0.312 W/mK, which is roughly 116.9% of that of pure PEG. For 2D nanostructure, Mehrali et al. ^[43] realized that, due to their high intrinsic thermal conductivity, a range of PCCs can be created by dipping PA in various commercially available PNGs (300, 500, and 750 m²/g). The obtained thermal conductivity of 91.94 wt% is 2.11 W.m⁻¹.K⁻¹ (eight times higher than the pure PA). For 3D nanostructure, Chen et al. ^[44] were the first to demonstrate a PCC in which CNTs were utilized as a supporting structure to contain PW and boost the thermal conductivity of the PCC. With 80wt% PW, the thermal conductivity of the PCC reached roughly 1.2 W.m⁻¹. K⁻¹, representing a nearly six-fold improvement over pure PW.

Figure 4. Illustration of the fundamental carbon nanostructures: (A) 1D carbon nanostructure (1D CNTs and CFs), (B) 2D nanostructure (graphene, graphite nanoplates, EG, and GO), and (C) 3D nanostructure (carbon foam, and graphite/graphene foams and aerogels) ^[41].

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